Investigating the development of midbrain dopaminergic neurons using mouse embryonic stem cell reporter lines
thesisposted on 06.02.2017, 05:49 by Su, Colin Tze En
Embryonic stem (ES) cells possess the capability to self-renew indefinitely and are capable of generating any cell of the three primary germ layers, making them an attractive source of material to investigate both basic physiological properties and neurodegenerative processes. Although ES cells can be directed into specific cell lineages, the differentiation of ES cells results in heterogenous cultures. To date, there are few differentiation protocols that produce homogenous populations of any desired cell type. Many methods have been used in an effort to obtain homogenous populations of cells; from forced expression of genes involved in developmental pathways, to FACS isolation of cells expressing markers of interest. There has been a considerable focus on generating homogenous populations of midbrain dopaminergic progenitors (or neurons) for Parkinson's disease which involves the degeneration of a specific population of midbrain dopaminergic neurons. In this thesis, I investigate the development of mouse embryonic stem cells into midbrain dopaminergic neurons using reporter cell lines. In the first experimental chapter, I investigate the expression of Lmx1a and Msx1; two key transcription factors implicated in dopaminergic neuronal development. I also examine the impact of the BMP, Shh and Wnt signalling pathways on dopaminergic neural differentiation. Activation of the BMP and Wnt pathways resulted in inhibition of neural induction and the expression of both Lmx1a and Msx1. In contrast, antagonising these signalling pathways increased the yield of tyrosine hydroxylase (TH) expressing neurons. Activating or inhibiting the Shh pathway did not affect Lmx1a, Msx1 or TH expression. These experiments show that early Lmx1a expression is not indicative of the number of dopaminergic neurons produced. Furthermore, many of the TH positive neurons derived from monolayer cultures were not of midbrain origin. In the following experimental chapter, I used immunocytochemistry and qPCR to characterise the population of cells expressing Lmx1a. The downstream targets of Lmx1a, Msx1 and Wnt1, and midbrain dopaminergic neuron markers, Lmx1b and En1, were significantly upregulated in Lmx1a positive cells. The Lmx1a positive fraction was enriched with neural progenitors, and give rise to highly neural cultures. However, the majority of neurons in the terminally differentiated cultures derived from Lmx1a positive cells were GABAergic. Immunocytochemistry identified these cells as forebrain GABAergic neurons with upper-layer identity. Furthermore, the isolated Lmx1a positive cells were not responsive to patterning cues, indicating that they were already committed towards a GABAergic neuron fate. To show that these Lmx1a+ progenitors could generate dopaminergic neurons I used an alternative differentiation paradigm, the PA6 co-culture method. Expression of Lmx1a in PA6 co-cultures was different from monolayer cultures; the percentage of Lmx1a positive cells increased throughout the differentiation period. In addition, PA6 co-culture derived TH positive cells were found to co-express Lmx1a, an occurrence that was uncommon in monolayer cultures. The ionotropic glutamate receptors on neurons derived on adherent monolayer and PA6 co-cultures were functionally characterised in the final experimental chapter. Previously, antagonism of ionotropic glutamate receptors has been reported to improve behavioural assay scores in Parkinsonian animal models (Johnson et al., 2009). Terminally differentiated monolayer cultures and PA6 co-cultures responded differently to stimulation with glutamate, AMPA kainate and NMDA. The ionotropic glutamate receptors of midbrain dopaminergic and GABAergic neurons derived from both culture systems were further investigated. An initial characterisation indicates distinct differences between the glutamate receptor populations in monolayer and PA6 co-cultures. It appears that monolayer differentiation generates AMPA expressing midbrain dopaminergic neurons in comparison to the NMDA receptors evident following PA6 differentiation. Interestingly, these differences in receptor expression appear restricted by culture method, rather than neuronal subtype, i.e. monolayer neurons expressed AMPA receptors, regardless of whether they were TH+ or GAD67+. Similarly both TH+ and GAD67+ neurons appeared to express NMDA receptors following PA6 differentiation. At present the significance of these findings is unknown. In addition, the effect of Wnt5a on cell responses to glutamate agonists was examined. Wnt5a was able to potentiate cell responses to sub-maximal concentrations of certain glutamate agonists depending on the differentiation paradigm performed.